The present invention relates to an apparatus and to a method for making fine powder of an alloy of two or more metals, and more particularly relates to such an apparatus and method for making alloy powder of two or more metals, of which the diameters of the alloy powder particles are on the order of some few hundreds of angstroms.
The present invention was orignally made in Japan, and the first patent application made therefor was Japanese Patent Application No. 81538/83, of which priority is being claimed in the present application; and the subject matter of that previous Japanese patent application is hereby incorporated into this specification by reference; a copy is appended to this specification.
Prior art methods of manufacturing fine powder, both of pure metals and of alloys of two or more metals, such as the type of fine pure metal or alloy powder used for sintering material and as dispersion material for making particle dispersion composite materials, have generally involved mechanically pulverizing solid masses of pure metal or alloy material, atomizing molten pure metal or alloy material, or colliding a stream of molten pure metal or alloy material with an object at low temperature; but the diameters of the particles of pure metal or alloy powder made by such prior art methods as above have typically been of the order of from ten to five hundred microns.
In general, the smaller are the diameters of the particles of a fine pure metal or alloy powder, the better is the pure metal or alloy powder for use as raw material for sintering or for making particle dispersion composite materials, because, due to and according to the increase in the total surface area of the particles of the metal or alloy powder relative to their total weight, in the case of sintering the higher the density of the resultant sintered material becomes, whereas in the case of making a particle dispersion composite material the better the mechanical properties of the composite material become, due to the relative increase in the importance of the forces due to their surface activity to the forces due to their mass. Therefore, it has been realized for a long time that it is very desirable to make fine pure metal or alloy powder with as small a particle diameter as possible, and many laborious studies have been conducted with this aim in view.
One method that has been experimented with for making very fine metal particles has been the vacuum vapor deposition method. In this method, a metal is heated in vacuum and is vaporized into vapor of its gas, and this metal vapor gas is then condensed on the surface of a low temperature object, i.e. a collector. Another method that has been attempted involves vaporizing a metal in a low pressure but not vacuum environment made up of an inert gas at a pressure of from a tenth to a hundredth of atmospheric pressure or thereabouts, so that the vapor of the metal is cooled by the inert gas so much as to be brought into the oversaturated state. Thus this metal vapor condenses into fine powder in either the liquid or the solid phase. This method, called the gas vaporization method, has been successful for producing small amounts of fine alloy powder on an experimental basis.
These methods have been successful in making fine metallic powder with average particle diameter less than one micron, but, since all of these methods make use of various gradual vapor condensation phenomena, there have inevitably been large fluctuations in the particle diameters of the alloy powder obtained (i.e. the standard deviation of these diameters has been great), and furthermore the rate of production of alloy powder has been extremely low. In order to improve the productivity of these methods, it appears to be necessary continuously to take out the generated metal vapor from the chamber in which it is generated, and to cool it. Therefore, there have been proposed methods in which the metal vapor is carried on a plasma flow to take it out of the metallic vapor production chamber, and is then cooled by striking it or colliding it against a collection plate, which may be a water cooled copper plate. Also, methods have been proposed in which the metal vapor is absorbed into a sheet of oil which is dripping down along a bearing member or the like, and again the metal vapor is condensed in this way. However, the former method involving the use of a plate for condensing the metal vapor requires large and expensive facilities, while the latter method of absorption into oil is not high in absorption effiency. Accordingly, up till the present date, it has been practically very difficult to mass produce fine alloy powder with very small and uniform particle diameter in an efficient and economical way.
A subsidiary troublesome problem that has occurred in the manufacture of fine pure metal or alloy powder is that, when the particle diameters are very small and when the powder is manufactured in vacuum conditions or in an inert gas atmosphere, the powder can have a tendency towards self ignition when it is removed and is brought into contact with the ordinary atmosphere, even at room temperature. This is because, as the particle diameter decreases, the surface area of the particles included in a given mass of alloy powder increases dramatically, and therefore the chemical activity of the particles increases. Therefore, in the prior art, it has been recognized to be desirable to perform post processing on fine alloy powder before removing it into the atmosphere from vacuum or an inert atmosphere where it has been formed, by forming under controlled conditions an oxide film on the surfaces of the particles. However, according to such conventional methods, this has increased the cost of the process, and has also lowered the quality of the finished product and its effectiveness for use as a sintering material or a particle dispersion material.
In a copending patent application Ser. No. 608,167, invented by the same inventors as the present application and assigned to the same assignee, which it is not intended hereby to acknowledge as prior art except insofar as constrained to do so by law, there were proposed a method for making fine powder of a metal, comprising the steps of: producing vapor of said metal; mixing a flow of inert gas with said vapor of said metal to produce a mixture gas; rapidly cooling said mixture gas by adiabatically expanding it by passing it through a nozzle; and collecting metal powder fromm a flow out from said nozzle; and a device for practicing the method, comprising: a vaporization chamber for producing metal vapor therein; means for heating said vaporization chamber; means for introducing a flow of inert gas into said vaporization chamber; an exit flow path from said vaporization chamber, comprising a nozzle therealong; a powder collection zone into which the flow out from said nozzle is directed; and means for evacuating gases from said powder collection zone. Thus, the present inventors discovered that, in the case of manufacturing fine powder of a pure metal according to that proposal, if as specified above the generated metal vapor is brought out from the zone in which it is made by directing it through a nozzle, then the rapid adiabatic expansion cooling provided to the metal vapor as it passes through the nozzle is very effective for causing the metal vapor to condense into extremely minute particles. Further, the present inventors discovered that, by mixing in a quantity of inert gas such as argon or helium for use as a carrier gas with this metal vapor, before passing the mixture through the nozzle for adiabatic expansion cooling, thereby the metal vapor can be introduced into the nozzle all the more quickly, and since thus the growth in the size of the metal particles resulting from the conglomeration together thereof is restricted, fine metal powder with even more even and consistent particle diameters can be made even more efficiently. Further, with the addition of this carrier gas, the adjustment of the temperature and pressure conditions before and after the nozzle can be made with very great facility, by controlling the flow rate of this inert gas, and hence the particle diameter of the resulting fine metal powder can be easily and closely controlled. Thus, since the metal vapor is prevented by the inert gas from undergoing particle growth through agglomeration, metal vapor is quickly and continuously and smoothly introduced into the nozzle as carried by the inert gas, and thus it is quite practicable to make metal powder with particle diameter of a few hundred angstroms or so in quantity.
Now, the above problems are relevant both to the manufacture of fine powder of a pure metal and fine powder of an alloy of more than one metal; but in practice the method and device of the proposal mentioned above are not suitable for making powder of an alloy of two or more metals. FIG. 1 of the appended drawings, in which the vapor pressures of various metals are shown on a logarithmic scale along the vertical direction, and temperature is shown on a logarithmic scale along the horizontal direction, illustrates that the vapor pressures of different metals drastically differ at the same temperature. For example, at 1800.degree. C. the vapor pressures of Al, Cu, and Ni are respectively 12 torr, 5.6 torr, and 0.3 torr. Also, the boiling points of different metals differ widely. Therefore, if in an attempt to apply the method and device of the proposal mentioned above to manufacture of, for example, fine alloy powder consisting of an alloy of aluminum and nickel, a mixture of aluminum and nickel were charged into the vaporization chamber and were heated, this would result in the evolution of first substantially only aluminum vapor, then a mixture of a lot of aluminum vapor and only a little nickel vapor, and then as the temperature rose and the aluminum became vaporized away from the percentage of nickel vapor would rise until the evolved vapor was substantially only nickel vapor; and a mixture vapor of aluminum vapor and nickel vapor of substantially the same proportional composition as the amounts of these metals charged into the vaporization chamber at the beginning would not be produced for any substantial time. Hence alloy powder of aluminum and nickel would not be satisfactorily produced with a consistent composition. Even if this could be managed somehow, as for example by constant addition of more of aluminum or nickel, the proportions of aluminum and of nickel in the produced alloy powder particles could not freely be determined at will according to the desired use of the fine alloy powder; but the vapor pressures of the aluminum and the nickel would exercise much undesirable constraint thereover.
Further, since in practice the actual demand for fine alloy powder made up of two or more metals is much greater than the demand for fine powder made of a single pure metal, due to the well known fact that in an alloy each constituent can supply deficiencies of others, this problem is a very important one, and a solution is very much needed.